Printed wiring members are commonly used as ways to electrically interconnect electronic components in low cost fashion. Some types of printed wiring members are designed for direct interconnection to semiconductor chips by wire bonding or tape automated bonding, for example. Furthermore, some types of printed wiring members are designed to have connector pads that are intended to disconnectably mate multiple times with an electrical connector that connects the printed wiring member with other circuitry in an electronic system. An example of a printed wiring member that both directly interconnects to a semiconductor chip and also has disconnectable connection pads is an inkjet printhead. In this example, the semiconductor chip is the inkjet printhead die, which typically contains the nozzles, the drop forming mechanisms to eject drops from the nozzles, and electronics associated with the drop forming mechanisms. Because printheads typically do not last the entire lifetime of the printer, many types of printheads are designed to be disconnectable from the printer to allow replacement.
Although nominally a printhead would only need to be installed and uninstalled once, it is preferable to design the printhead connector pads to withstand more than 10-20 installation cycles in order to improve reliability of the system. One well-known way of making connector pads reliably connectable for many cycles is to provide a top metallization of hard gold on the connector pads. Hard gold (around 99.7% pure) is made hard during the plating process by adding cobalt and/or nickel at levels of approximately 0.1% to 0.3%. As is well-known in the art, however, hard gold is not readily wire-bondable. In order to provide a printed wiring member with bondable gold at the contact pads and wear resistant gold at the connector pads, one approach would be to do two separate masking and plating steps to provide soft gold (around 99.9% pure) at the contact pads and hard gold at the connector pads, but this is relatively costly.
U.S. Pat. No. 5,910,644 discloses metallization electroplated onto the copper contact pads and connector pads of a printed wiring member having both bondability and wear resistance. The disclosed metallization includes 80-200 microinches (2-5 microns) of nickel plated onto the copper, nominally 35 microinches (0.9 micron) of palladium plated onto the nickel, and 5-30 microinches (0.1-0.75 micron) of soft gold plated onto the palladium. The high purity soft gold provides for high yield wire bonding while the palladium ensures adequate wear resistance to provide stable electrical connection in the event that wear through of the soft gold surface finish of the connector pads can occur. A drawback of this process is that relatively thick layers of the costly palladium and high purity gold are required.
Consequently, a need exists for a method of making a printed wiring member that provides reliable bondability, wear resistant connector pads for repetitive printhead installations, and low cost. Although an inkjet printhead is a familiar example of such a need, there are other examples in the chip-on-board industry of the need for a method of making a printed wiring member that provides reliable bondability, wear resistant connector pads for repetitive component installations, and low cost.